The present invention relates to a satellite broadcast tuner, and in particular, to a satellite broadcast tuner which receives both an analog satellite broadcast (FM (frequency modulation)) and a digital satellite broadcast (QPSK (Quadrature Phase Shift Keying) modulation).
A prior art satellite broadcast tuner of this type will be described with reference to FIGS. 7 through 11. FIGS. 7 and 8 are schematic views of satellite broadcast receiving systems which receive both the conventional analog satellite broadcast (FM) and the digital satellite broadcast (QPSK modulation).
In the satellite broadcast receiving system shown in FIG. 7, a digital satellite broadcast receiver 203 and an analog satellite broadcast receiver 204 which are independent of each other are connected to independently provided parabola antennas 201 and 202, respectively. The digital satellite broadcast receiver 203 includes a digital satellite broadcast tuner 205, while the analog satellite broadcast receiver 204 includes an analog satellite broadcast tuner 206. The reference numerals 207 and 208 denote input terminals of the receivers 203 and 204, respectively.
Further, the parabola antennas 201 and 202 are each provided with an LNB (Low Noise Block down converter (not shown)) to be used for the digital satellite broadcast or the analog satellite broadcast.
When receiving a satellite broadcast with the above structure, RF (radio frequency) signals that have been down-converted to a 1-GHz band by the LNBs of the parabola antennas are independently inputted to the input terminals 207 and 208 of the receivers 203 and 204. Then, the digital satellite broadcast receiver 203 executes QPSK demodulation, while the analog satellite broadcast receiver 204 executes FM signal demodulation.
The satellite broadcast receiving system shown in FIG. 8 is an example in which a parabola antenna and an LNB are in common used for the digital satellite broadcast and the analog satellite broadcast. Components having the same functions as those of FIG. 7 are denoted by the same reference numerals. In the structure of FIG. 8, the digital satellite broadcast receiver 203 is connected to one parabola antenna 209 (provided with the LNB), and the digital satellite broadcast tuner 205 inside the digital satellite broadcast receiver 203 is further provided internally with an RF distributor 210. Then, the digital satellite broadcast receiver 203 and the analog satellite broadcast receiver 204 are connected with each other by connecting an RF output terminal 211 provided at the digital satellite broadcast receiver 203 to an input terminal 208 provided at the analog satellite broadcast receiver 204.
With the above arrangement, when receiving the digital satellite broadcast, the RF signal that has been down-converted into the 1-GHz band by the LNB mounted to the parabola antenna 209 is inputted from the input terminal 207 and then subjected to QPSK demodulation by the digital satellite broadcast receiver 203.
On the other hand, when receiving the analog satellite broadcast, the RF signal of the analog satellite broadcast that is similarly inputted to the input terminal 207 is distributed by the RF distributor 210 and then transmitted from the RF output terminal 211 to the input terminal 208 of the analog satellite broadcast receiver 204. Then, the FM signal is demodulated by the analog satellite broadcast receiver 204.
FIG. 9 shows a detailed structure of the analog satellite broadcast tuner 206. In FIG. 9 are shown a high-pass filter 222, a first RF amplifier 212, an attenuator 213, a second RF amplifier 214, a low-pass filter 215 and a down-converting first mixer 216. The reference numeral 217 denotes a first local oscillator circuit for supplying a local oscillation signal to the first mixer 216. The first mixer 216 outputs a first intermediate frequency signal having a frequency equal to a frequency difference between the RF signal and the local oscillation signal.
The reference numeral 218 denotes a PLL (phase-locked loop) circuit for locking the frequency of the local oscillator circuit 217 based on a channel data given from a microprocessor (not shown). The first intermediate frequency signal outputted from the first mixer 216 is amplified in an IF (intermediate frequency) amplifier 219, thereafter limited in bandwidth by a SAW (Surface Acoustic Wave) filter 220, and then transmitted to an IF.multidot.AGC (automatic gain control) amplifier 221. Then, the first intermediate frequency signal amplified in the IF.multidot.AGC amplifier 221 is frequency detected by a PLL type FM signal detector 231 to output a base band signal.
Further, an AGC circuit 230 is provided between the IF.multidot.AGC amplifier 221 and the attenuator 213. The AGC circuit 230 controls the attenuator 213 and the IF.multidot.AGC amplifier 221 according to the input level of the FM signal detector 231, the input level detected by an AGC detector 232.
It is to be noted that the reference numeral 223 denotes a low-pass filter, and the reference numeral 235 denotes a changeover switch for the SAW filter 220.
FIG. 10 shows a detailed structure of the above digital satellite broadcast tuner 205. Components having the same functions as those of FIG. 9 are denoted by the same reference numerals. In FIG. 10, the structure from the high-pass filter 222 to the IF.multidot.AGC amplifier 221 is approximately equal to that of the analog satellite broadcast tuner 206, and therefore, description is omitted therefor.
The reference numeral 400 denotes an I/Q quadrature detector which has two second mixers 401 and 402 for second down-converting use. It is to be noted that the reference numeral 216 denotes the first mixer for first down-converting use. The IF signal that has passed through the IF.multidot.AGC amplifier 221 is distributed into two paths to be inputted to the two second mixers 401 and 402 of the I/Q quadrature detector 400. The reference numeral 403 denotes a second local oscillator circuit which is a fixed-frequency local oscillator that oscillates at a frequency approximately equal to the intermediate frequency. Then, an output of the second local oscillator circuit 403 is distributed into two local signals having a mutual phase difference of 90.degree. by a 90-deg. phase shifter 404. Thereafter, the resulting signals are inputted to the second mixers 401 and 402, where they are each mixed with the IF signal to be converted into base band signals.
The resulting I and Q signals that have been converted into the base band signals are amplified in base band amplifiers 224 and 225 and limited in bandwidth by Nyquist filters 226 and 227, and thereafter the resulting signals are outputted from an I-signal output terminal 228 and a Q-signal output terminal 229, respectively. Thereafter, the signals are inputted to a QPSK demodulation circuit in the subsequent stage.
FIG. 11 is a block diagram of the above QPSK demodulation circuit to be connected to the I/Q quadrature detector in the above digital satellite broadcast tuner.
The I/Q signal that has been obtained through the I/Q quadrature detection in the digital satellite broadcast tuner 205 is inputted to a QPSK demodulation circuit (video and audio processing circuit) 240 as shown in FIG. 11. Then, in this QPSK demodulation circuit 240, the I/Q signal is put through an error correcting section comprised of a QPSK demodulator 241, a Viterbi decoder 242 and a Reed-Solomon error correcting section 243, and thereafter the resulting signal is distributed into two paths to be transmitted to circuits in the subsequent stage. One of them is outputted as a video signal via an MPEG (Moving Picture (Coding) Experts Group) video decoder 244 and an NTSC (National Television System Committee) modulator 245. The other is outputted as an audio signal via an MPEG audio decoder 246 and an audio digital-to-analog converter 247.
Further, a control signal from the QPSK demodulator 241 is inputted to an input terminal 234 of the AGC circuit 230, and the AGC loop is controlled so that the I/Q signal at an appropriate level is inputted to the QPSK demodulation circuit 240.
It is to be noted that the reference numeral 248 denotes an M-COM (Microcomputer) and the reference numeral 249 denotes a DRAM (Dynamic Random Access Memory).
According to the satellite broadcast receiving system shown in FIG. 7, a total of two sets of the parabola antennas 201 and 202, LNBs and satellite broadcast receivers 203 and 204 are necessary for digital use and analog use.
On the other hand, according to the satellite broadcast receiving system shown in FIG. 8, only one set of the parabola antenna 209 and LNB is necessary, however, two sets of the satellite broadcast receivers 203 and 204 are still necessary.
In either case, wiring of the total system is complicated, and this disadvantageously leads to cost increase.